So I believe I have figured out why Yoast Sitemaps are incompatible with wpForo (Version 2.3.1) and there have been multiple reports of this critical error issue (example: https://wpforo.com/community/general-forums-bug-reports/26039/#post-75400)

The file /wpforo/classes/SEO.php has a regex that is falsely catching the /page-sitemap.xml URL.

In /wpforo/classes/SEO.php see line 344 in the public function "redirect"

	public function redirect( $current_object, $wpf_url_parse ) {
		$pattern = '#^[\r\n\t\s\0]*(?<type>\w*)-?sitemap(?:(?<paged>\d*)|_index)\.xml[\r\n\t\s\0]*$#iu';
		if( ! empty( $wpf_url_parse[0] ) && preg_match( $pattern, basename( (string) $wpf_url_parse[0] ), $match ) ) {
			$current_object['template'] = 'sitemap';

			$query = [ 'type' => '', 'paged' => 1 ];
			$query = wpforo_parse_args( $match, $query );
			$type  = trim( (string) $query['type'] );
			$paged = absint( $query['paged'] );
			if( ! $paged ) $paged = 1;

			if( $sitemap = $this->get_sitemap( $type, $paged ) ) {
				$this->output( $sitemap );
				$this->sitemap_close();
			} else {
				$current_object['is_404'] = true;
			}
		}

		return $current_object;
	}

The regex in question is below:

#^[\r\n\t\s\0]*(?<type>\w*)-?sitemap(?:(?<paged>\d*)|_index)\.xml[\r\n\t\s\0]*$#iu

If you run a regex testing tool with that regex against the following string, you'll see that it matches it, and then proceeds to load the forum template. The URL page-sitemap.xml is Yoast's page sitemap, so wpforo should not be running any code for this url.

page-sitemap.xml

I believe the proper solution is to update this regex in the core wpForo plugin's code to something more precise so it does not override the yoast sitemap.

=== 

The error we are receiving in full is below for reference.

PHP Fatal error: Uncaught Error: Call to undefined function wpforo_thread_forum_template() in /nas/content/live/sitename/wp-content/plugins/wpforo/themes/2022/layouts/4/forum.php:85\nStack trace:\n
#0 /nas/content/live/sitename/wp-content/plugins/wpforo/themes/2022/forum.php(41): include()\n
#1 /nas/content/live/sitename/wp-content/plugins/wpforo/includes/functions-template.php(2196): include('/nas/content/li...')\n
#2 /nas/content/live/sitename/wp-content/plugins/wpforo/themes/2022/index.php(26): wpforo_template('forum')\n
#3 /nas/content/live/sitename/wp-content/plugins/wpforo/includes/hooks.php(230): include('/nas/content/li...')\n
#4 /nas/content/live/sitename/wp-includes/shortcodes.php(433): wpforo\\wpforo->{closure}(Array, '', 'wpforo')\n
#5 [internal function]: do_shortcode_tag(Array)\n
#6 /nas/content/live/sitename/wp-includes/shortcodes.php(273): preg_replace_callback('/\\\\[(\\\\[?)(wpforo...', 'do_shortcode_ta...', '[wpforo]\\n')\n
#7 /nas/content/live/sitename/wp-content/plugins/so-widgets-bundle/widgets/editor/editor.php(108): do_shortcode('[wpforo]\\n')\n
#8 /nas/content/live/sitename/wp-content/plugins/so-widgets-bundle/base/siteorigin-widget.class.php(194): SiteOrigin_Widget_Editor_Widget->get_template_variables(Array, Array)\n
#9 /nas/content/live/sitename/wp-content/plugins/siteorigin-panels/inc/renderer.php(835): SiteOrigin_Widget->widget(Array, Array)\n
#10 /nas/content/live/sitename/wp-content/plugins/siteorigin-panels/inc/renderer.php(1195): SiteOrigin_Panels_Renderer->the_widget(Array, Array, 2, 0, 0, true, true, 45, '')\n
#11 /nas/content/live/sitename/wp-content/plugins/siteorigin-panels/inc/renderer.php(1163): SiteOrigin_Panels_Renderer->render_widget(45, 2, 0, 0, Array, true)\n
#12 /nas/content/live/sitename/wp-content/plugins/siteorigin-panels/inc/renderer.php(1072): SiteOrigin_Panels_Renderer->render_cell(45, 2, 0, Array, Array, Array)\n
#13 /nas/content/live/sitename/wp-content/plugins/siteorigin-panels/inc/renderer.php(553): SiteOrigin_Panels_Renderer->render_row(45, 2, Array, Array)\n
#14 /nas/content/live/sitename/wp-content/plugins/siteorigin-panels/compat/yoast.php(44): SiteOrigin_Panels_Renderer->render(45, false)\n
#15 /nas/content/live/sitename/wp-includes/class-wp-hook.php(324): siteorigin_yoast_sitemap_images_compat(Array, 45)\n
#16 /nas/content/live/sitename/wp-includes/plugin.php(205): WP_Hook->apply_filters(Array, Array)\n
#17 /nas/content/live/sitename/wp-content/plugins/wordpress-seo/inc/sitemaps/class-sitemap-image-parser.php(127): apply_filters('wpseo_sitemap_u...', Array, 45)\n
#18 /nas/content/live/sitename/wp-content/plugins/wordpress-seo/inc/sitemaps/class-post-type-sitemap-provider.php(664): WPSEO_Sitemap_Image_Parser->get_images(Object(WP_Post))\n
#19 /nas/content/live/sitename/wp-content/plugins/wordpress-seo/inc/sitemaps/class-post-type-sitemap-provider.php(211): WPSEO_Post_Type_Sitemap_Provider->get_url(Object(WP_Post))\n
#20 /nas/content/live/sitename/wp-content/plugins/wordpress-seo/inc/sitemaps/class-sitemaps.php(379): WPSEO_Post_Type_Sitemap_Provider->get_sitemap_links('page', 1000, 1)\n
#21 /nas/content/live/sitename/wp-content/plugins/wordpress-seo/inc/sitemaps/class-sitemaps.php(279): WPSEO_Sitemaps->build_sitemap('page')\n
#22 /nas/content/live/sitename/wp-includes/class-wp-hook.php(324): WPSEO_Sitemaps->redirect(Object(WP_Query))\n
#23 /nas/content/live/sitename/wp-includes/class-wp-hook.php(348): WP_Hook->apply_filters('', Array)\n
#24 /nas/content/live/sitename/wp-includes/plugin.php(565): WP_Hook->do_action(Array)\n
#25 /nas/content/live/sitename/wp-includes/class-wp-query.php(1881): do_action_ref_array('pre_get_posts', Array)\n
#26 /nas/content/live/sitename/wp-includes/class-wp-query.php(3824): WP_Query->get_posts()\n
#27 /nas/content/live/sitename/wp-includes/class-wp.php(696): WP_Query->query(Array)\n
#28 /nas/content/live/sitename/wp-includes/class-wp.php(816): WP->query_posts()\n
#29 /nas/content/live/sitename/wp-includes/functions.php(1336): WP->main('')\n
#30 /nas/content/live/sitename/wp-blog-header.php(16): wp()\n
#31 /nas/content/live/sitename/index.php(17): require('/nas/content/li...')\n
#32 {main}\n thrown in /nas/content/live/sitename/wp-content/plugins/wpforo/themes/2022/layouts/4/forum.php on line 85